Several methods for manufacturing powder coatings have been reported in trade publications and in the patent literature. Among these are melt mixing, spray drying, and coagulation. Microscopic examinations of powders produced by these techniques show that the shape of the particles thus produced is such that no dimension thereof is significantly dominant. Spray drying and coagulation both tend to yield, in predominant fraction, smooth, substantially round, powder particles. By way of contrast, melt mixing tends to yield, in predominant fraction, particles having sharp edges. While all of these methods are relatively suitable for the production of powder coatings which do not contain metal flakes as one of the particulate pigments, they have been found generally unsuitable for the production of powder coatings which do contain metal flakes. This unsuitability is manifest by an improper orientation of the metal flakes when such paints are applied to the substrate by electrostatic spray techniques, the most common method of applying automobile finish coatings.
In liquid metallic paints, the orientation of the metal flakes is primarily parallel with the surface of the coating film or tilted so as to form a small angle with such surface. While the orientation of a particular flake may form an angle with the surface of the film, it is an essential characteristic of a good metallic coating that the net orientation of all such flakes be essentially parallel to such surface.
When the methods above mentioned are used to prepare resin coated metal flakes for use in powder paints they yield resin particles in which the metal flakes within a particle are oriented randomly. When such a powder is applied to a substrate and baked to form a coating, the orientation of the metal flakes in the film is influenced by the direction of flow of the powder during baking. For example, when a powder coated surface is baked vertically there is a net flow of the coating downward and parallel to the surface. Consequently, a large fraction of the metal flakes is oriented in a downward angle to the horizontal, thereby producing a coating in which the brightness varies substantially with the angle of observation. In particular, the coating is much brighter when observed with the direction of flow than it is when observed opposite the direction of flow, an undesirable phenomenon, known in the trade as "reverse flop". On the other hand, when a powder coated surface is baked in horizontal alignment there is no net bulk flow parallel to the surface and the orientation of a fraction of the metal flakes in the coating remains perpendicular to the surface of the coating film. Since that fraction does not reflect light, the brightness of the coating is much less than that of a corresponding liquid metallic paint or a powder metallic paint that has been baked in vertical alignment.
Metal flakes are almost always employed in combination with non-metal, pigment and together they impart a colored metallic appearance to an automobile body or other article painted. Since the orientation of the metal flakes also influences the color development, or hue, of the coating, there is a noticeable difference in that respect between horizontally and vertically baked powder coated surfaces of the prior art. In addition, the metal flakes, which are oriented perpendicular to the surface of the coating film, tend to protrude through the surface of the coating, thereby reducing gloss. Consequently, the gloss of horizontally baked surfaces of such prior art is lower than that of corresponding vertically baked surfaces.
A method for producing powder paint which involves release of a liquid paint formulation into water is disclosed in U.S. Pat. No. 3,737,401 to I. H. Tsou. Three methods are disclosed by this patent. In the first, the liquid paint is agitated in a container into which is rapidly poured a large amount of coagulating liquid. This is continued until coagulation of the non-solvent (i.e., film-forming) portion of the paint is completed. The patentee states therein that this method is suited to batch processing and that it is difficult to control the sizes of powder particles resulting from this method. The second method disclosed is to agitate a container of the coagulating liquid and slowly pour the prepared paint into the agitated second liquid. The patentee states therein that this method is suited to batch processing, produces a somewhat narrower range of powder particle sizes than the first method, and that it is difficult to control particle size using this method. The third method, patentee's preferred embodiment, is suited to continuous production and permits a greater control over particle size and particle size distribution. In this method, the liquid paint is introduced into the coagulating liquid beneath the surface of the coagulating liquid near the mixer blade where the agitation of the coagulating medium is at a maximum. Thus, all of these methods contemplate intermixing of the paint solution and the coagulating liquid under conditions of agitation and the preferred and most effective method calls for introducing the liquid paint at the point of highest agitation. The powder paint particles thus formed from minute drops of liquid introduced into high turbulence inherently tend toward the spherical shape. If these particles contain metal flakes, the orientation of such flakes is inherently random.